This invention relates to a protection system for an electrical power network, and more particularly, to a pilot wire relay protection system for detecting and responding to phase-to-phase, three-phase, phase to phase and to ground, and ground-current faults including relatively low level ground current faults occurring within the electrical power network.
Various pilot wire relays derive their information of the condition of a protected line of a high voltage transmission network from current transformers each having their primary winding arranged across a portion of each high voltage transmission line which is typically conducting one phase of the power transmission network. Such a pilot wire relay is described in a General Electric Instruction Book GEH-1811C, published by General Electric Company, Power Systems Management Business Department, Philadelphia, Pennsylvnia, 19142. Current transformers arranged across the high voltage transmission lines may be subjected to heavy currents that occur during faults within the systems at which time the current transformers may become saturated and their output may not correctly represent the condition of the transmission lines. Furthermore, faults within a high voltage distribution system may cause excessively high currents to flow within the transmission lines. The relatively large current may cause, by residual magnetism, the cores of the current transformers to be placed in a magnetized state after the current is removed from the system. The magnetized condition of the current transformers may in turn cause the transformers to provide an erroneous output signal representative of a faulted condition even though the magnitude of the current flowing within the transmission line is representative of a non-faulted condition. In certain power systems, typically industrial systems, it is common practice to limit the maximum ground fault current by the use of a power resistor. The power resistor is typically connected to the neutral of the high voltage power network and limits the maximum current occurring during a ground fault in an industrial system, to a value typically in the order of 400 amperes. However, current transformers coupled to a transmission line with this ground fault limiting resistor may still provide erroneous signals and thus limit the desired protection for the power network.
The present invention provides an arrangement of current transformers which is substantially less susceptible to the erroneous effects produced by relatively high current flowing within an electrical distribution network.
Accordingly, an object of my invention is to utilize this novel arrangement of current transformers, having reduced error susceptibility to high current faults, as the input source to a pilot wire relay protection system to correspondingly provide more accurate protection for electrical distribution network.
Another object of my invention system is to utilize this novel arrangement of current transformers as the means for allowing a reduction to the minimum fault current detectable as a ground fault condition and to permit the design of electrical distribution networks wherein the maximum ground fault current has been limited to values lower than heretofore possible while still providing rapid detection of ground faults.
Still further, it is another object of my present invention to provide a pilot wire relay having analog and electronic means to supply an improvement to the accuracy at which the plot wire relay protection system determines the condition of the protected portion of the electrical network.